The present invention relates to an optical head tracking control device and, more specifically, to an optical head tracking control device which enables a stable track-lead-in operation even with an optical disk rotating at high speed and at the occurrence of a track deviation due to disk eccentricity.
A conventional tracking control device which enables a stable track-lead-in operation even at the occurrence of a track deviation due to eccentricity of an optical disk or some other factor is disclosed in Japanese Unexamined Patent Publication No. Sho 61-194645 (1986), for instance. In this device, the speed of a light spot when it traverses a track is detected based on a signal indicating a track deviation, and a pulse signal indicating that the light spot has just passed the track center is generated based on the signal indicating the track deviation and a signal indicating the quantity of reflection light from the optical disk. A track-head-in operation is started by closing the tracking servo loop when a light spot has just passed a track center at a track-passing speed lower than a predetermined speed.
However, the data transfer rate has increased in the recent video disk field, for instance, 140 Mbps is required for digital-recording a non-compressed PAL video signal. To write this signal onto a disk at a current recording density, the disk needs to be rotated at a speed as high as about 4,500 rpm, which is more than two times higher than the conventional speed. On the other hand, to elongate the recording time, it is also required to reduce the track pitch of a disk. For example, while the track pitch of the conventional compact disk (CD) is 1.6 .mu.m, employment of a 1.2 .mu.m track pitch is now being considered in the video disk field.
In the conventional tracking control device disclosed in the above-mentioned publication, the tracking servo loop is closed when a light spot has just passed a track center at a track-passing speed lower than a predetermined speed. However, under the exacting conditions, such as a disc rotation speed of 4,500 rpm and a track pitch of 1.2 .mu.m, which recent video disks are required to satisfy, it is difficult for the tracking servo control to effect, in a stable manner, a track-lead-in operation even if it is started at a time instant when a track center is detected. This is due to the fact that since the lead-in start point is the track center, only half of the overall track width is available after the start of the lead-in operation. In addition, currently, there exist optical disks having an eccentricity of several hundred micrometers. A track-lead-in operation is more likely to fail with a disk having such a large eccentricity.